Three-year retention of radiactive caesium in the body for TEPCO workers involved in the Fukushima Daiichi nuclear power station accidentThree-year retention of radiactive caesium in the body for TEPCO workers involved in the Fukushima Daiichi nuclear power station accident

As a response to the Fukushima Daiichi nuclear power station accident, National Institute of Radiological Sciences (NIRS) received seven Tokyo Electric Power Co.(TEPCO) workers who were suspected to be heavily exposed over 250 mSv (including internal exposure mainly from 131I and external exposure) and has continued health examination for these workers over three years. NIRS has been also performing direct measurements with a whole-body counter (WBC) to monitor the retention of radioactive caesium (Cs-134 and Cs-137) for the workers. The WBC used is a highly-sensitive measuring system with a low-background shield chamber made of 20-cm-thick steel and six large HPGe detectors. It was previously reported that the effective half-life for radioactive caesium was ~ 100 days on average of the workers, which was in good agreement with the Cs biokinetic model proposed by Internal Commission on Radiological Protection (ICRP). However, the discrepancy appeared in measurement data in the third year, showing a component with longer effective half-life. This discrepancy was also observed in all the workers whose data were obtained. Although this component would not give a large l impact on the present dose estimations, further additional measurements are needed to clarify the biokinetic behaviour of radioactive caesium in the late phase.国際学会IM2015(International Conference on Individual Monitoring of Ionising Radiation)